The present invention relates to a method and apparatus for forming lapped-seam cans and, more specifically, to such a method and apparatus in which an organic adhesive film is stuck onto end portions of a can blank so as to coat said end portions and thus the coated can blank is formed into a can.
The coating of the end portions of the can blank with such an organic adhesive film to prevent iron from being dissolved into the content of the can is very important, for instance, for the preservation of the flavor of soft drinks and to prevent the hydrogen swelling of soft-drink cans in which malic acid or like substances that accelerate iron dissolution are filled.
For this purpose, at least one end portion of the can blank forming the inner side of the formed can must be coated with an adhesive film stuck thereto so as to prevent pin-hole formation. However, even with a nylon type film having a relatively high mechanical strength, it is very difficult to stick a film having a thickness of 40 to 100 microns and a width of 6 to 10mm onto the end portion of the can blank moving at a speed of 70m/min. or faster in such a manner that neither pin-holes nor wrinkles are formed.
Heretofore, for example, U.S. Pat. No. 3,807,332 has proposed a method of coating the end portions of a can blank with an organic adhesive film in which almost a half portion of the film is first stuck onto one side of the blank, having cut-burrs thereon as cut from its original blank, and, then, the remaining half portion of the film is sequentially folded back onto the other side of the blank. However, in such a conventional method, since the organic adhesive film is folded back under the guidance of the end portions of the can blank which are preheated and have sharp cut-burrs, pin-holes are often formed in the film. Further, according to this prior art method, there is a disadvantage in that, since the adhesive film must be always kept under tension in order to cause the film to be uniformly stuck onto the can blank, an internal stress is produced in the tape-shape adhesive film and, as a result, wrinkles will be formed due to dislocation of the film after being stuck onto the blank.
U.S. Pat. No. 3,816,206 has disclosed an alternative method in which the end portions of the can blank are first coated with adhesive extruded from an extruding machine and, after the formation of the can, the exposed iron surfaces are coated through an inside striping process. However, this method not only adds to the number of processes, but can be said to be far from complete in respect of iron surface coating.
Among the conventional can forming methods and apparatus in which an adhesive is stuck onto the end portions of a can blank, there have been proposed, for example, a method and apparatus in which an adhesive is extruded and stuck in a ribbon-shape onto one end of the can blank, and the blank having one end thereof stuck with the adhesive is formed into a cylindrical shape and, then, the adhesive is heated to be melted by a heating means such as a gas burner so that the overlapped end portions of the blank can be joined together under pressure. However, in such a method and apparatus that end portion of the can blank forming the inner side of the lapped portion of the formed can cannot be coated with the adhesive and, therefore, must be subjected to an inside stripping in a later process.
In addition, U.S. Pat. No. 3,912,568 discloses another method in which a can blank having one end portion thereof coated with an organic adhesive film is first formed into a cylindrical shape, and the thus-formed can blank is transferred in a sliding manner along a cylinder which is liquid-cooled substantially over the entire length thereof, during which the end portions of the blank are heated by applying a well-known high-frequency induction method as disclosed in British Pat. No. 644,839 and, finally, said end portions are overlapped, joined one on the other, and cooled by means of a liquid-cooled hammer, so as to form a can. However, in this method according to U.S. Pat. No. 3,912,568, since the end portions of the blank slide against the feeding guide when the organic adhesive film on the end portion of the blank is transferred along the cylinder, pin-holes are produced in the film, thus additionally damaging the coating effect at the cut end portions of the blank. Further, since the can blank formed into a cylindrical shape which is being transferred in a sliding manner is cooled substantially over the entire length of the cooling cylinder, the can blank is fully cooled except for the end portions thereof during the transfer of the blank in contact with the liquid-cooled cylinder. On the other hand, only the end portions of the can blank are subjected to the high-frequency induction heating for melting the adhesive film. As a result, a great temperature difference is produced between the thus heated end portions and the remaining portions of the can blank, which causes the can blank to be distorted in a corrugated fashion. If the end portions are overlapped and joined one on the other by removing such a distortion with a press or a like means, it follows that the overlapped end portions will be stuck together with an internal strain left in the adhesive layer and uneven forces will be exerted on the adhesive layer sections, so that those sections having a weak adhesive strength will be peeled off, leading to leakage of the can content.
In order to transfer can blanks formed into a cylindrical shape by means of feeding pawls, there must be a certain interval between successively fed can blanks. However, if the can blanks are subjected to the high-frequency induction heating with such an internal left between successive blanks, the edge effect, namely, an effect that the temperature of the corner portions of the can blank becomes higher than that of other portions when subjected to the high-frequency induction heating, is remarkably enhanced as compared with a case in which the can blanks are fed more closely one after another. This edge effect produces not only a variation in the temperature condition of the adhesive melting, but the uneven temperature distribution also causes the blank to be distorted.
To prevent such an uneven temperature distribution from occurring, some modification or improvement on the configuration of the high-frequency heating coil may be considered. However, from a practical point of view, it is very difficult to execute various configurations for a cylinder of small diameter.
In general, a crystalline polymer material such as nylon is used as an adhesive for joining the seam of a lapped seam can. In such nylon type adhesives, the degree of crystallization varies with the heating and cooling conditions. Thus, since the adhesive strength and workability of nylon is greatly affected by the degree of crystallization, it is difficult to manufacture cans having uniform quality without controlling the heating temperature, cooling temperatures and cooling time of the nylon adhesive so as to maintain a constant degree of crystallization in the adhesive after being cooled and cured. However, even high-frequency induction heating cannot be free from some variation of heating temperature. Therefore, in order to cure the adhesive at a constant degree of crystallization, it is necessary to measure and control the temperature of the adhesive on the end portions of the can blank and the temperature of the cooling hammer in a continuous manner.
In the prior art systems, since the can blank is initially formed into a cylindrical shape, the temperature measurement at the end portions of the can blank is almost impossible. On the other hand, according to the method of the present invention, in which the can blank is transferred as a flat plate to a point immediately before its formation into a cylindrical shape, the aforementioned temperature measurement and controlling can be conducted.
Accordingly, an object of the present invention is to provide a method in which a tape-shaped adhesive can be stuck onto the end portions of a can blank without an inclusion of bubbles in such a manner that the end face of the end edge of the blank as well as the upper and lower sides of the end edge portion can be coated uniformly and positively with the adhesive in a single step process.
Another object of the present invention is to provide a method and apparatus for forming cans in which the positioning of cans to be transferred is facilitated, in which the film of adhesive stuck onto the end edge portions of the can blank will not be peeled off, in which the edge effect due to a high-frequency induction heating as well as the corrugation phenomena (distortion) of the heated blank are effectively prevented from occurring, and in which the degradation of the adhesive strength of the lapped portion can be avoided after the formation of the cans.
A further object of the present invention is to provide a novel method of press-forming the end portions of the can blank for preventing micro-leakage phenomena from occurring in filled cans.